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What would a pilot do if a medical emergency occurs slightly further from the destination than the nearest diversion?
Imagine a long-haul flight and that a passenger on board starts suffering from a heart attack.
At that very moment, they notice that they are 3 hours away from where they came and 3.$m$ hours, such that $m$ is some positive number, away from their destination. Also, assume that there is no other place that pilots can land other than the airports located at where they go and they came.
What are the pilots supposed to do?

$m<1$ go on, $m>1$ return. What exactly is the question? Why are you doubting that a pilot will not go to the closest suitable airport in case of a life threatening emergency?

@Sabschises Firstly, thank you for your answer. Secondly, I am in fact not doubting, but just trying to figure out whether pilots take the likelihood of survivability of the passenger experiencing a health problem necessitating immediate attention into account while making a decision. Now, another question is even though $m$ can be less then one it is still positive and I do not see why its being less than 1 matters.

Welcome to aviation.SE! I'm not sure what you're asking here, because the pilots and crew aren't doctors and may not be able to determine accurately what's wrong with the passenger and therefore what the "survivability" is. Are you just asking if pilots always divert to the nearest airport, all other things being equal?

@ Pondlife. Well, let me rephrase and generalize a bit. We are going to B from A and we are in such a situation that we are, say, 2 hours away from both A and B. Suppose that there are only two options. We can either go back to A or proceed B; the only difference is that we well know that it would take,say, 1 fewer minute to reach back to A than B. What should we do? In fact, the question is, if we will gain 1 hour by returning home, then it is clear that home is where we are going to divert. But, what if gain is minimal and still exists.So, basically, I am curious about the threshold, if any.

Rephrased, short: "When approximately equidistant between two airports, how do pilots decide where to divert?"

$m=0.1$ means it's 18 only minutes to return to the airport, and that's still a positive number. But you seem to be specifically asking about $m$ close to 1.

@Sanchises: He means 3.0 hours one direction, and 3.1 hours the other direction. (eg: close to the midpoint)  You only got 18 minutes if you multiplied 3 hours x 0.1.

If it's a medical emergency such as heart attack 10 minutes extra on 3 hours is nothing, you're stuffed regardless of which way you go. As others have said, you're rarely that far from help and if it's a true emergency the pilot will divert to wherever will provide help the soonest.

There is no hard-and-fast rule for what to do in an emergency, including a medical emergency involving a passenger.
The decision also is not an instantaneous one; while the flight crew evaluates and works on the medical emergency, the plane is constantly getting closer to the destination.
I suspect that when 3 hours from help, an extra 10, 20, or 30 minutes is unlikely to definitively change the outcome, and the pilots would likely choose to continue to the destination.   If returning is 45 minutes or more closer than continuing, then returning to the origin starts to look pretty good.
The facilities available at each destination might factor into the decision as well:  If they left Chicago OHare, and are enroute Port-a-Prince, Haiti, then its very likely that there are better medical services at Chicago.
Ultimately, its a judgement call dependent on many, many factors.

BTW:  There are relatively few places in the world where an aircraft can truly be 3+ hours from any airport.  It could happen in the South Pacific and southern Indian Ocean, but not many other places.

@ abelenky As I said that was just a hypothetical question but you just taught me something I did not know, so many thanks! cheers.

It’s also likely that in that scenario, crew will contact a station such as medlink who will be able to give medical advice. On longer flights with several hours flying time to nearest alternate, the aircraft will be equipped with resuscitation kits, and cabin crew will have some medical training, too.

Just as a supplement to @abelenky's good answer, there are some things to be taken into account that haven't yet been mentioned.
First, if you've been flying with a tailwind for 3.0 hours and the winds would be roughly the same, it's going to take you more than 3.0 hours to fly back along the same route, possibly a lot more than 3 hours.
Second, technically you can't just reverse course without getting a clearance to do so. You have to contact ATC and negotiate the course reversal. That could take several minutes, possibly a lot more depending on communications. The captain could, of course, decide to reverse immediately and negotiate the clearance later, but he would be risking a possibly dangerous reduction in separation from other traffic, and I doubt whatever the governing authority was would take kindly to risking a couple of plane loads of people for the sake of one individual.
Then, there can be special circumstances. For example, if you were flying Indonesian Muslim pilgrims to Mecca on a Hajj charter. I can't speak to current policy as I retired in 1999, but back then the practice was that once you were in the air, you did not turn back or divert for medical emergencies. There was always a doctor on board, and technically he could request a captain to divert to the nearest suitable, but in practice that was never done. It was expected that there would be occasional deaths, and we carried body bags for that contingency.

Can you explain the reason behind this policy ?

@Antzi You can't get 200,000 pilgrims from Indonesia to Mecca in a matter of days if you accommodate diversions, and since the typical Indonesian pilgrim is elderly, there will be health problems. Plus, the belief is, as I understand it, that if you die while performing the Hajj, that's a direct entry to heaven. That apparently makes a death less problematical. I'm an atheist, not a Muslim, so I'm just parroting what I was told when I asked the same question you have.


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How do fighter jets communicate with an intercepted plane that's lost comms?
RAF Typhoons were scrambled today to intercept an Antonov 26 that lost comms, triggering a sonic boom over London.
The flight was escorted to Stansted airport, and the answer at What's the point in escorting a threatened flight with two fighter jets?
 says they "guide" the rogue jet to the airport.
How do they actually do this? Once they've established the jet is not a threat, how do they communicate with the other pilots?
I have an image in my head of them writing ← 120° (etc.) on a little whiteboard :P

This is covered in the FAA's AIM, Chapter 5, Section 6. The UK has similar procedures.
First, the standard guard frequency is 121.5 or 243.0 MHz. If you aren't listening to this frequency and get intercepted, you should start listening.
Methods like LED signs or possibly as whiteboard as you suggested can also be used. Otherwise, they maneuver their aircraft to send standard signals. See the guides above for details on the signals sent by interceptors and how they should be acknowledged. Basically:

You have been intercepted, follow me

Interceptor: Rocks wings (and flashes lights irregularly at night)
Intercepted: Same

You may proceed

Interceptor: Breakaway, climbing turn away from aircraft
Intercepted: Rocking wings

Land here

Interceptor: Circling airport, lowering landing gear, overflying runway (landing lights on at night)
Intercepted: Lower gear, land

The intercepted aircraft can also send signals. The interceptor will respond with one of the signals above.

Designated airport is inadequate

Intercepted: Raise gear, flash landing lights

Cannot comply

Intercepted: Switching on and off all lights, but distinct from flashing lights

In distress

Intercepted: Irregular flashing of all lights

In the case of the Washington, DC Special Flight Rules Area (DC SFRA), there is a Visual Warning System (VWS) of red and green lights that will let the pilot know that they shouldn't be there. The pilot is supposed to turn directly away from the center of the SFRA. If they are in contact with ATC they must advise that they have been illuminated by the system, otherwise they should contact ATC on a guard frequency and identify themselves.
Of course, an interceptor can't force an intercepted aircraft follow instructions. It goes on to explain what will happen if you don't follow those rules, which probably apply in any situation where the risk is too high:

Further noncompliance with interceptor aircraft or ATC may result in the use of force.

What happens if the intercepted flight is a neer-do-well with bad intentions, but follows the above signals?  At what point will it be shot down?  What reasons would a pilot have to not comply?

@Scottie how would they find out?

@Scottie I'd guess there's procedures in place to handle deviations from issued orders (like, target stops following for 20 seconds, doesn't respond to new orders, etc.).

@Scottie - If the pilot of the intercepted aircraft is compliant with orders of the intercepting aircraft, then there is no reason to suspect ill intent. The interceptor will usually lead the intercepted aircraft as far away from inhabited areas and aircraft traffic patterns as it is feasible to do enroute to a divert field, so our ne'er-do-well pilot isn't going to have much chance to do real damage with his plane without deliberately disobeying an instruction and thus tipping off the interceptor.

There are "rules of engagement" for interceptors in various situations, taking into account the severity of the violation for which the aircraft is being intercepted as well as the potential for harm to others as a result, so the question of when they would take the step of blowing a pilot out of the sky is situation-dependent. Nick the edge of DFW's Class B bubble in a 172 and you'll get a talking to when you land. Fly directly into the DC FRZ at 250 feet ignoring comms, and you're likely to get a very close look at a Raptor or F-16 within seconds, especially if the President's home.

Lastly, the only two scenarios I can think of where an aircraft would at least appear to ignore an interceptor's instructions are (1) a non-pilot at the controls with the PIC incapacitated (as is fantasized in so many movies and books) who doesn't understand the meaning of the intercepting aircraft's signals, or (2) a complete electrical system failure rendering the pilot unable to signal with the lights (they can still signal with wing rocks etc). Neither are likely, but they aren't impossible.

I heard of a case where a small aircraft had a compass/gyro failure and was flying out to sea.  It had to be intercepted (by another private/commercial plane, not military) and guided back to land.  However, the interceptor was unable to fly as slowly as the small plane did, and whenever he had to circle around to maintain visual contact, the small plane kept turning back towards the heading his broken gyro indicated instead of holding course…

AIM section 6 describes what the pilot in the intercepted plane should do:
First thing the intercepting craft will do is rock the wings irregularly (and flash the nav lights at night) and initiate a turn (usually on the left). The intercepted aircraft should copy that and follow. That signals that an intercept has taken place and the other plane is compliant.
If time is of the essence then the fighter can do a climbing turn in front of the other aircraft in the direction it should turn.
If the escort is at an end (and allows the craft to fly on) then the fighter will break away with a 90° turn, the response is to wiggle the wings again and continue on.
To signal the intercepted plane to land the fighter will circle the airport and do a flyover over the runway with lowered gear (imitating a go-around). The other plane may do a flyover to make sure it's clear to land and then go land.
If the intercepted pilot believes he can't land on the runway he should stow his gear and flash his landing lights during the flyover.
It's possible the intercepting plane will hand over control to the tower and let it use the light gun to guide it down.
If at any point the pilot cannot comply he should switch on and off his lights at a regular interval. If the intercepted pilot is in distress then he should flash all available lights

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Has a supersonic airplane with a truss-braced wing been studied/developed?
The X-66 truss braced jet is to be transonic. But have any published studies or development programs been done for a supersonic equivalent?
Seems like someone considered it, given that it allows reducing wave drag without grossly increasing weight or wetted area.

Not sure that wave drag would be lower...

Not exactly but on a related note check out a couple of designs (that never flew) from over a decade ago:  SAI's QSST and DARPA's QSP.

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Drag breakdown of a typical civil airplane wing
How much of the drag produced by a typical GA/airliner/cargo plane's wing at cruise is induced, frictional, and pressure drag?

Induced drag is mainly due to pressure drag. It'd be more correct to ask how much is induced and how much is parasite

At best L/D, an aircraft (with a simple parabolic polar) has half induced and half parasite drag.
If half the parasite drag is from the fuselage -- and you are flying at best L/D, then we can say that of the total drag:
25% fuselage parasite
25% wing parasite
50% induced
Or, of just the wing
33% parasite
66% induced
You likely aren't cruising exactly at best L/D.  And you will have other components that contribute to the parasite drag of the aircraft.  But this should give you a reasonable idea of how to make a starting estimate.


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How does GPS measure any plane's ground speed?
How does GPS measure an equipped airplane's ground speed assuming that the plane in question actually uses GPS as opposed to an inertial navigation system or other method?
(This was originally part of another question)

It's quite strange that the determination of the aircraft ground speed using a GPS is off-topic. It's a trivial question that may be closed, but not with the off topic criteria imho. VTR.

Could be a duplicate of this more general question on ground speed though.

@fooot I don't think it is, and since there is no question with a very similar title, considering the simplicity of the question, I decided to go ahead and get it knocked out on here.  Nothing says a question isn't allowed to be simple, or obvious.

From casey's answer to that question, Comparing two GPS derived locations will yield a velocity vector and ground speed is its magnitude.

So, is SE beginning to institute the marking of duplicate answers then?  This is not the same question.

@fooot It doesn't matter whether casey's answer applies here or not, we can't start marking things as dupes because of overlapping answers. See the comments on this question for a clearer example of why - in my opinion - it's a bad idea. Personally, I'd rather see 5 short, simple, obvious questions with equally simple answers than one 'generic' question that might answer everything if you read through all the answers.

@Pondlife per meta If you ask a question similar to another question and it is likely to get the exact same answer, you have yourself a duplicate question. imo, ground speed in general is not that much more complex and sufficiently covers the GPS part, there is no reason to answer it again.

What we're saying here is that an answer to How is the actual ground speed of an airliner measured?, in short, could be "by using GPS technology."  However, "How does GPS technology measure ground speed" is not answered by, "By using GPS technology."  Although some of the information may indeed overlap, this simplifies the search for knowledge by having a more targeted question with a more targeted answer.

A good practice is to leave the following format of comment:  "Related: How is the actual ground speed of an airliner measured?

Following the close vote's logic here, "How does an inertial navigation system measure a plane's ground speed?" would also be a duplicate, even though IMHO it isn't at all.  This is really about indexing properly as to not create a wild goose chase for answers.

Exactly the same way that your car GPS measures your ground speed.

(Source)
The receiver can evaluate the location at two successive times, and simply compute the velocity vector from differentiation. This gives a precision of several meters per second.
This velocity can be derived from the Doppler shift of the L1 carrier frequency. This true measure is far more accurate, up to centimeters per second, but is less simple to implement, as the frequency shift must be smoothed over a period of time.
Reference: How does a GNSS receiver estimate velocity?

I you want to relate the speed of the aircraft to the ground you have to taken into account that an aircraft is flying at 37000ft parallel to the earth. So with a bit of trigonometry you can come up with a ground speed.

@Brilsmurfffje: That's a good point! The difference between sea-level and 37,000 ft circumferences is about 70 km only, about 0.2%. Not that much, but you are right.

Not quite, in the air the actual distance travelled is not the same as distance travelled on the ground. A correction has to be made for altitude.

GPS is primarily made to determine the position of the receiver.
Therefore, GPS uses several satellites with exactly known positions which send very precisely timed signals. From the time difference between the signals when arriving at the receiver, the own position in space can be calculate, typically given in earths coordinate system.
Get the position two times, calculate distance, divide by time difference, and you have speed.
Another way is to measure the frequency of the signals from the satellites, since it slightly varies with speed due to the Doppler effect. This is similar to the shift in pitch of the horn of a police car passing you.
This method gives you a speed value directly with your position.
As side note:
GPS often is not that precise and gives your position with some offset. BUT the offset is mostly constant, so while the position is not correct, the distance traveled and so the speed is.

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Are municipal airports in the U.S. treated differently than most public places in regards to legal, holstered, loaded firearm carry?
Is a municipal airport treated like a normal public place which requires signage or registration with a no-carry list to ban firearms from being carried on the premises, or...
... is a municipal airport treated like a government facility such as a courthouse, post office, or DMV office?
Or is there some other rule regarding guns that are loaded, holstered or otherwise carried on or about a person?
Airport terminals designed primarily for part 121 airline operations which are actively screened by the Transportation Security Administration are beyond the scope of this question.
Laws vary from state-to-state, so if carry is not legal in those states, then this question is not being asked in regards to those states, obviously.
Related: Are US pilots permitted to carry & use their firearm in countries where guns are illegal?

Hmm, I do know people who have taken their guns out for a fly, so at least some airports allow them.

@SMSvonderTann I have read in the past that it is legal, but I was hoping for something a little more solid.  I wonder if landing in a state where it isn't legal that one had intended to fly-over, but due to an emergency had to stop, would get someone arrested.

There is no rule. With the exception of military airfields, regular airports are not owned by the federal government, nor are they federal property in any way. State laws apply.

I've landed at an airport in Texas that had a sign on the terminal door positively encouraging you to bring your gun... I believe the FBO sold ammo


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Is there any significance to the flap design on some Cessna citations?
In some business jets, I've noticed that the flaps seem to be split into sections. Here is one example on Cessna citation
As you can see, the flaps are split into three sections. The Embraer Phenom also has this design but this is because of the planes sweep make requiring the gap.

But the citation has a straight wing, so wouldn't it be better to have one piece flaps like the Gulfstream G650?

If you look at the flap track system carefully on the Latitude, you can see the track system being used occupies the space between the panels when retracted, and that particular configuration required splitting the panels to accommodate the tracks, probably because the vertical space requirement for the track system made it easier to fit in if the panels were split into separate segments. They may have also decided to split the segments up for ease of manufacture of the segments (smaller jigs), and/or for easier/cheaper replacement in the field.
The G650 has a similar internal hidden track system free of vanes or canoes fairings, but they were able to squeeze the track dimension down so that the bottom of the panel could be continuous, gaining a bit of aerodynamic efficiency. The cost may have been tracks that took up less vertical space, but were heavier as a result.
Give different groups of engineering monkeys a particular set of alphabet blocks, and when each group finally throws their blocks in a way that spells a word, each group will have a different word.

I cannot find any good sources describing the Lattitude flap track system. Do you happen to know any?

No. I could more or less discern it from the pics. Flap systems were a specialty of mine when I was in the tech support business. It'll be something like the tracks in a Cessna 172, except really fancy machined forgings with big rollers, etc.  No small feat to package it all to fit inside the wing mold line.


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What is a Integrated Avionics Unit (IAU) and is it related to a IMA?
Reading through a TBM 930 avionics handbook, I came across the integrated avionics unit. What does this do and how does it work? Also, why does the auto pilot only follow one flight director as seen on the text?

What do you expect to happen if an autopilot were to attempt to "follow" two flight directors that were giving something besides the same commands?

Not terribly familiar with the G3000 integrated flight deck components, but, at a guess, an Integrated Avionics Unit (IAU) is most likely the “brains” of the system, taking in air data, magnetometers and user inputs from the touchscreen controllers and AFCS unit.  It has its own AHARS and contains all of the operating components, radios, GPS, transponder, etc. and operates the two PFDs and MFD accordingly.

The Integrated Avionics Unit (IAU) is the core component of the Garmin G3000 setup used in the TBM 930. For redundancy, there are two IAUs. They contain various radio and GPS receivers and interface with several inputs (like ADC, AHRS) and outputs (like PFD, autopilot servos):

GIA 63W (2) – The Integrated Avionics Units (IAU) function as the main communications hub, linking several
LRUs with each on-side GDU 1200W. Each IAU contains a GPS Satellite-Based Augmentation System (SBAS)
receiver, a very high frequency (VHF) communication/navigation/glideslope (COM/NAV/GS) receiver, a Flight
Director (FD) and system integration microprocessors. The IAUs communicate with each other through using
usign HSDB and additional backup paths.

(Garmin G3000 Integrated Flight Deck Pilot’s Guide for the TBM 930 - 1.1 SYSTEM DESCRIPTION)
The two flight directors (one in each IAU) also exist for redundancy. Each pilot will see the flight director command bars of their respective side IAU on their PFD. Usually, they should agree with each other. In case of differences, the pilots need to determine which unit functions correctly and can select the active flight director using the XFR button. The autopilot servos can only be connected to one flight director input at a time because these inputs might differ and the servos wouldn't know which one to follow.
This is different from an autoland system on larger aircraft, which allows multiple autopilots to be connected at the same time. The TBM 930 does not have autoland capability and therefore does not need such a complex setup.

Thanks for all the information!


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Is ISA deviation always zero at the stratosphere?
I watched a video of a pilot who stated that the ISA deviation is always near zero when at the stratosphere, is this true? And if so, what is the science behind it?
Here is the video

ISA Model
In the International Standard Atmosphere (ISA) model, the temperature at sea level is set at 15°C and decreases at a lapse rate of 2°C per 1,000 feet. This lapse rate is maintained up to Flight Level (FL) 360, where the temperature stabilises at -57°C. Beyond FL360, up to approximately 65,000 feet or 20 km, the temperature remains constant at -57°C in the ISA model (see Note 1).
Stratosphere Meteorology
The stratosphere exhibits a relatively stable temperature profile, particularly in the lower levels. This is fundamentally different from the troposphere. In the troposphere, temperature generally decreases with altitude due to the adiabatic expansion of rising air. In contrast, the stratosphere's temperature remains relatively constant, largely owing to the presence of the ozone layer. This layer absorbs ultraviolet (UV) radiation from the Sun, releasing heat in the process. This heat offsets the cooling effects of radiation emitted back into space, resulting in a balanced temperature profile.
The Rule of Thumb
Given that the ISA temperature at FL360 and above is -57°C, and the stratosphere also maintains a constant temperature of approximately -57°C, the ISA deviation would indeed be zero. This holds true whether you're at FL370, FL400, or even higher altitudes. Hence, the rule of thumb mentioned in the video—that a zero ISA deviation likely indicates you're in the stratosphere—is generally accurate.
Limitations
However, there are limitations to this. For instance, if you experience a consistent ISA deviation of zero throughout your climb, it becomes challenging to discern when you've transitioned into the lower stratosphere's constant temperature zone. Additionally, the lower stratosphere temperature close to the tropopause can vary (see Stratosphere Temperature Forecasts) due to factors like solar cycles, volcanic activity, and latitude. This rule of thumb may be less reliable outside the latitudes above 65° N/S - due to both a lower tropopause and typically colder stratosphere temperatures.
This rule of thumb will break down when the actual tropopause is lower then FL360 as the following example illustrates.
Scenario: Flying at high latitudes in the winter, the tropopause is at FL290.
The ISA model suggests a temperature of -43°C at this level. If the actual temperature is -57°C, your ISA deviation would be -14°C. The negative deviation indicates you're still in the colder troposphere, when in fact you might well be in the stratosphere.

Note 1: The lapse rate stopping at FL360 in the ISA Model is commonly referenced in aviation meteorology textbooks and across the web (for example: https://skybrary.aero/articles/international-standard-atmosphere-isa). Many places, like Wikipedia, reference ICAO Doc 7488 Manual of Manual of the ICAO Standard Atmosphere for this, however this document doesn’t actually state this fact. I have been unable to provide a source for this but is generally accepted.

Adding to some of the confusion is that technically the flat temperature lapse exists only in the tropopause, which is the boundary between the troposphere and the stratosphere. In the stratosphere the temperature actually rises at a rate of 0.3 °C per 1,000 feet. However for general purposes the tropopause is considered the lower level of the stratosphere, and since the stratosphere's temperature lapse above the tropopause of +0.3 °C is relatively small compared to the troposphere's -1.98 °C per 1,000 feet, the stratosphere is often described as one layer having a flat temperature lapse.

I couldn't find an ISA source but the U.S. Standard Atmosphere 1976 table 4 on page 3 shows from 0-36K feet (0-11 km) a lapse rate of -1.98 °C/1000ft (-6.5 K/km), from 36K-65K feet a lapse rate of 0, and from 65K-105K feet a lapse rate of +0.3 °C/1000ft. Interestingly the word tropopause does not appear anywhere in the document, however the table seems to indicate that the USSA model  tropopause begins at 36K feet, stratosphere at 65K, upper stratosphere at 105K feet, stratopause at 154K feet, mesosphere at 167K feet.


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Why this Mooney is draining fuel when priming
Today at the flying club, I witnessed a Mooney startup. When the pilot primed the engine, there was a fair amount of fuel (I’m assuming) that was drained near the nose gear. What is the reason for this? And how does it work?
https://youtube.com/shorts/O8dYQgQTrq8?si=ryKy-_Hezas7Gxdb

It appears to be an M-20R which has a Cont IO-550. As a fuel injected engine, you prime it by advancing the mixture momentarily to rich while the fuel boost pump is running. This dumps fuel from the main injectors into the intake ports, rather than have separate primer injectors like on a carbureted engine.
You advance the mixture for a certain amount of time to spray fuel in, say, 3 seconds or some such, and back to idle cutoff. Then crank the engine and move the mixture up when it catches.  If you prime too much, raw fuel starts running down the intake runners and into the airbox below the throttle body, that directs either primary or alternate air into the intake. The airbox will have a drain line for this purpose, so that raw fuel is dumped overboard instead of collecting in the airbox and possibly leaking out in the engine bay, creating a fire risk.
Injected engines are quite easy to overprime because it's controlled by counting in your head and you may need more or less depending on how warm or cold it is outside. So it's a bit of an art.

Just a comment in general, and not about your answer, specifically. In this day and age, where safety of the populace has taken paramount, seems like having something like priming with fuel be 'an art' and not a manufactured-in process, is ... well ... less than safe. I'm sure the number of flight-line fires caused by priming fuel overflow is astronomically low compared to the total number of engine starts, but still. Having even one GA aircraft burn up is too many, especially to that plane's owner.

Problems is the alternatives, chokes, are not used, mostly because they are a failure risk, so priming is it, and absent electronic fuel injection, it has to be done manually, as nearly all airplanes are carbureted or have mechanical FI. Fuel pissing out the cowl from overpriming is not that common an occurrence, b/c you have to really overdo it, it's still a fire risk, and if an airport manager saw that Mooney dumping asphalt dissolver on the pavement like that, the Mooney driver would know about it. When I say it's an art, it's mostly about learning not to overdo it when an engine is hot.


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Why do some aircraft's flight control surfaces not droop when hydraulic power is taken away?
Analyzing a bunch of different photos of airliners sitting at Gates, I've noticed that some have flight controls that droop (due to no hydraulic power) and sometimes they stay leveled. Why is this?

In the first image of the Japan airlines Boeing 787, the rudder and elevators clearly have no hydraulic power. Yet the ailerons are still leveled. In the second picture of the West jet Boeing 787, there seems to be no flight control droop.
This isn't just about the 787 but all aircraft in general.

@Bianfable I've looked at that question and it doesn't really answer my question.

I can't see how the questions are different. If you think they are you need to explain why to differentiate them.

@GdD I was asking why the same plane would have two different situations when one droops and the other doesn't.

I don't see control droop on either of those pictures.

@GdD The elevator on the top photo are drooped.

It doesn't look that way to me @Boeing787, but I'll defer to others on this one.

There are two kinds of Power Control Units used on flight controls. The PCU is the hydraulic cylinder and input control valves and linkage that drives the surface, normally two or three per surface. The input linkage operates a hydraulic spool valve that controls flow of fluid to either side of the cylinder, like in any piece of construction equipment.
Some PCUs, when you take hydraulic pressure away, open an internal valve that connects both sides of the actuator together. Also called an "idle circuit". These actuators are free to move when unpressurized, like a car shock absorber. They still have a damping function when in this state, which is important to flutter resistance for control surfaces that are not mass balanced, typical on jets with hydraulic controls.
On the ground, when all the hydraulic systems are unpressurized, a flight control will sag under its weight as it displaces the (usually 3) hydraulic actuators that are "idled" by the removal of pressure. They usually won't sag all the way to the bottom, because the mechanical input linkage starts to resist displacement of the cylinder after it moves some amount.
Other PCUs don't use an idle circuit and if there is no control input to the PCU, fluid is trapped in both sides of the cylinder and it's locked hydraulically in whatever position it's in. The surface can be made to move when unpressurized, but only if the PCU's control valve is diplaced by moving the control from the cockpit.
On the CRJs, the aileron PCUs (two per surface) are non-idling. When unpressurized, the ailerons are rigid at neutral as if bolted in place. However you can move them if you move the control wheel to displace the internal hydraulic input valve.
However the Elevator (and rudder) PCUs are idling type, and when unpressurized, the elevator surface will slowly sag about half way down until it starts trying to backdrive the mechanical linkage due to the input linkage geometry.

Is their a benefit to the rudder and elevators being the idling type vs non idling type?

I don't recall the specific reason one or the other is used. I think it allows you to take credit for the unit as a flutter damper when it's hydraulic system is unpressurized.

so would you say that one of those photos is a plane receiving maintenance services? Cause the other 787 seems to have all the flight controls locked in place.

The WestJet one has elevators sagging a bit as well. You can see if you zoom in.

There is a third type that requires positive flow to move. Compare my neighbor's John deer tractor to my iFarmall. When shutdown he can lower the hydraulics with the valve control, and given a day or two it will leak through the valve and settle on its own. But my Farmall will stay elevated indefinitely, altering the control valve position does nothing without pump flow. I have positive down pressure (beyond gravity), he has lift pressure only but that allows float (can be good for smoothing). Anyhow the Farmall has been parked over 6 months with elevated hydraulics, no sag.

That a non-idling actuator.  In the airplane the cylinder and control valving and input linkage are all in the same unit.

The control surfaces do not droop during power or hydraulic shut off technically because the isolation or switching valves switch to their default state which blocks pressure and return ports, thus the actuators are locked in position.
When the controls droop, the iso or switching valves port pressure to return, thus the flaps fall under gravity.
Why some planes are in the "droop" state while at the gate might be a maintenance reason. If you are doing maintenance you want the hydraulics powered down (pressure vented to return, thus drooped). My best guess.


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Why would a plane opt out of flying through a class bravo?
High performance planes that depart Chicago Executive are always vectored to the left of departing towards O'Hare airspace. They than loop around the airport until above O'Hare airspace and continue with their flight. Here is a picture of one example.

Why wouldn't they just fly through the airspace?

As a Low Hour Private Pilot, some Bravos are WAY more busy and complex than others.  I'll avoid the Dallas Class B if at all possible, but am comfortable inside other ones (MCI, SEA).  That is a Pilot Choice.  But in your scenario, it looks like its probably an ATC choice.

The better title for the question might be, "Why don't controllers vector aircraft through the Class Bravo?" since, flying IFR, where the vectors take you is the controller's call, not the pilot's (within reason... you don't have to take a vector into the middle of the thunderstorm, for instance).

To pile on, the premise of your question is incorrect. Whether flying IFR or VFR, all operations in Class B airspace are subject to an ATC clearance. The pilots are not "opting out" of flying through the Bravo; they are complying with ATC, and ATC does not want them flying through the Bravo.

@abelenky I also notice that students at many flight schools also avoid Class B airspace for some particular reason. It even goes up to the commercial applicant level and it is pretty funny to me. I see so many XC flight plans where the route goes in a mile wide gap between a restricted and a class B. It is kind of funny in my opinion.

@SMSvonderTann Students aren't allowed into class B as I understand stand from federal aviation regulations.

@Boeing787: Students aren't allowed in B until signed off by instructor. And certainly any commercial applicant should already lower-level certs and be Bravo-approved.  My only point is that some recreational/hobbyist pilots who are technically allowed in B might want to avoid it anyway, because it can be a challenging space.

@757toga I see, the plane was departing from Chicago Executive.

@Boeing787 I am more of referring to pilots who are full-time students getting instruction from a flight school. A lot of them already have their private, commercial with instrument ratings and are still afraid of getting in or near a Class B airspace even though they are legally allowed to. I notice this more in 141 or high-traffic 61 schools where students there get very little flight experience outside of their training.

@Boeing787 What altitudes do the different colors correspond to? Class B extends up to 10000 ft, so it's possible they are still flying through the Bravo airspace.

@mmathis The plane was at 17000 when they were passing over O'Hare airspace.

I've added another flight path with the same intent of waiting untill above O'Hare airspace.

The short answer: principally to keep other traffic not going directly into or out of KORD separate.  A jet like that headed south east bound would probably get the MONKZ5 departure which holds them at 3000 ft until cleared to their filed altitude by ATC.  If it’s a busy day at the Chicago TRACON, I could see sending a jet north like that to climb above the Bravo, thence vectored direct to MONKZ and thence on to their filed route of flight.  That routing would give the crew a minimum of time in the class, bravo area, and simplify separation between arrivals and departures into O’Hare and those coming from satellite airports in the Chicago area.
Staying out of the Class Bravo is also very popular for VFR departures and arrivals, simply because it avoids the aggravation of dealing with ATC to transit that area.  The bravo is really a sterile area set up to deal with high speed IFR arrivals and departures of jet aircraft. Any other kinds of traffic are a second thought and the controlling TRACONs typically route lighter aircraft, either outside of the bravo, or with minimal invasion into the bravo. Quite often times, general aviation pilots will either fly under the bravo shelves, or avoid the place altogether, simply because they don’t like the hassle of dealing with ATC and doing so expedite their ability to traverse the area.


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Why some airliners have autostart and others don't?
So some of the more modern FADEC equiped planes (specifically airliners like the 787) have autostart and will self monitor the engines during startup. The one thing that confused me was how a 747 could have auto start, and another 747 wouldn't have it. Is the auto start built into FADEC or is there a separate component necessary? Also, why doesn't the 737 NG have Auto start?

The simple answer is that not all models of the 747 are equipped with FADEC. The aircraft was first introduced in 1968. FADEC-1 was introduced in 1988. It would make sense that all 747's made during those initial 20 years would not have it.


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Are squawk codes linked with flight info such as route and flight data?
So squawk codes are given to an aircraft prior to departure. We know that if the pilot inputs the wrong code, the data will not show up. So are squawk codes linked via computer to the aircraft’s flight data, so once the pilot inputs the correct code the data will show up? Also, what is the process of requesting a code change? Does atc have to re link the new code?

In the united-states, the squawk code is the (primary) method of correlating a radar target with useful information such as the aircraft identifier, its destination and route, its requested altitude, etc, etc. It is possible for a controller to manually associate a radar track with a given flight, but this requires extra work and may cause issues with automated data transfer to other facilities. (Note that, unlike in Europe, the ADS-B-entered "flight ID" is not used for identifying a target; only the traditional Mode A squawk code is used.)
In other words, yes! The squawk code is "linked" with the rest of the information that stored in the flight data computer.
It is possible for ATC to manually assign a specific discrete squawk code to a flight, but the problem is that there is no immediate conflict warning if that code is already assigned to a different flight. So, almost without exception, we simply let the computer assign the code.

When an IFR flight plan is filed, the ATC computers have all the relevant flight information such as type of aircraft, destination, and suggested routing.
When the flight plan is opened, an available squawk code in the sector where the flight is located is assigned.   At that point the pilot would be expected to set the code in the transponder.
If that code is seen by the ATC radar, then the display will be able to find the filed and open flight plan and can display data from that plan.  If a different code is seen (perhaps because the transponder was mis-set), then the intended information won't be found.

what is the process of requesting a code change?

As the transponder code is just a (seemingly random) set of four digits, I'm not sure why a pilot would want a code change.   Can you give more information on what scenario might arise to ask for such a change?

Thanks for the info! I’ve seen videos were pilots have requested a code change due to superstitious reasons. For example, a squawk code of 4666.


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Why TBM 850 checklist calls for high idle during taxi
According to the TBM 850 checklist, at 13% NG you introduce LO Idle. But before taxi all TBM 850 pilots select high Idle. I was under the impression that high idle is selected for takeoff. Anybody know why it’s different than most other turboprops where you taxi on low idle? What exactly happens when the condition lever is selected from lo idle to high idle?

Isn't LO idle in feather, and high idle in ground Beta?  You need control over blade angle so you need to be in ground Beta to taxi.

@JohnK so I guess the different techniques are used by different manufactures?

Just about all turbo props with free turbines run in feather at ground idle and and are just inside beta mode in flight idle. Beta mode means blade angle is not controlled by the prop governor, but directly by the power lever, like a helicopter's collective.  You taxi in beta, with the ability to move the prop through forward, disc, and reverse pitch to go forward or backward.


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Why do some planes cage their attitude indicator while parked?
I've noticed that some planes cage their attitude indicators when Parked and uncaged them after start. As seen in this Lear 35 checklist. Why is this?

@757toga I've seen it on fighter jets and older model learjets as well. I put this photo just to give people an idea.

@757toga I've updated the picture if it makes more sense.

I don't see (and have never seen) a checklist item for caging after shutdown / when parking. Is this just an assumption on your part, based on the startup action? The startup action is just a check: obviously the AI won't work if caged, so it's important to check that it's not before flight. (On most [all?] AIs, a flag will be displayed when caged.) The presence of the check doesn't imply an expectation, in the same way "circuit breakers... checked" doesn't imply that the circuit breakers should be pulled after shutdown.

@TypeIA it says to make sure standby attitude indicator is off and caged at shutdown https://www.google.com/url?sa=i&source=web&cd=&ved=0CAMQw7AJahcKEwig99v9pbX7AhUAAAAAHQAAAAAQAg&url=https%3A%2F%2Ftomgorski.com%2Fasr%2FLear%2FLR35_Checklist_ATI.PDF&psig=AOvVaw0bGavFEmCJXiWR2w1jegto&ust=1668777277355921

@Boeing787 OK, then you're correct, and that's new to me. I look forward to the answer to why.

@757toga I uploaded it

I thought caging after parking protects the AI from movement/jarring while being towed.

If not caged the gyro can wobble and precess quite drastically while spinning down and could damage it. So: cage before letting it spin down and uncage only after it’s been spun up and erected.  Same before doing extreme aerobatics in some planes.

@Jim this looks like an answer!

The standby attitude indicator on the Lear 35 has its own power switch; it's not automatically energized when the battery masters come on.  Therefore it doesn't spin up on its own when power is applied to the rest of the aircraft.
If it's uncaged before it spins up, it could be damaged, and unlike the other gyros, it doesn't begin the spin up process until you flip the switch. So you power it on and leave it caged until it's up to speed, then you uncage it.  Similarly during shutdown, you cage it first and then remove power so that it can spin down safely.
A - Standby attitude indicator
B - Caging knob
C - Power switch

Original image by Don Popp, found here. Image is in the public domain.

Would this be the case for any plane?

@Boeing787 - I doubt it.  I haven't seen this on other airplanes, so I assume whatever mechanism a normal gyro uses to protect itself while spinning down, the Lear 35 standby gyro doesn't have.


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Supersonic propellers for supersonic aircraft?
A propeller is a spinning wing. Supersonic wings work. So, provided that the blades are made wide-chord and thin like supersonic wings are, with a supersonic airfoil, can they not power efficiently a supersonic aircraft?

"1.The supersonic regions at the blades create tremendous forces which will tear the propeller apart. Thus the design needs to be such that these forces can be mitigated. This is also the case for jet engines, which is why the ducts are designed such that the airflow into the engine is slowed down to subsonic speeds." 1.even supersonic wings experience tremendous forces. 2.propfans are a thing and may have supersonic blades, yet do better than turbofans. The second reason stated is irrelevant.

In case someone gets this misconception, I'm not asking why supersonic propellers don't exist, rather, what it would be like if they did. (I'm writing this here because this is where people will post links to "similar" questions)

One word: Thunderscreech.

Perhaps getting a little carried away with question trimming and pruning here.  Some good answers out there, and supersonic props do work, they are just very noisy.  Decibel limit regulations stopped the prop-fan for now.  And enclosing a very rapidly rotating unit for failure containment should keep the fan jets going for a while.  But, importantly, the faster you go, the more a prop blade needs to be pitched to get optimal AOA.  A little vector analysis shows you get less and less forward thrust the faster you go.  As some point it isn't worth the drag.

If you want to know just how noisy a supersonic propeller can be, read this artcle about the Republic XF84H. The engine could be heard 25 miles away, and ground crew suffered headaches, nausea, incapacitation and in one report, a seizure.The USAF ordered it off base because the noise was too disruptive.


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Is it true that blowing cannot be used for laminar flow control? Why?
Aerospace Engineering Desk Reference, page 505

Tangential blowing can only be applied to the prevention of separation...The flow caused by blowing tends to be very vulnerable to laminarturbulent transition...

Is this true? Why? Is it because of the high speed of the blowing jet? Is it not possible to use a slow-speed (but faster than the local flow) jet to circumvent this?

Yes, it is. What would a slow speed jet accomplish? Laminar flow means that all airflow is in one direction without crossflow. Any jet of different speed will disturb that calm, orderly flow, causing turbulence. And adding energy will only be possible with a faster jet.


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Why is the thrust-specific fuel consumption much worse for high bypass turbofans at cruise?
General Electric GE90
Takeoff: 0.278 lb/lbf/h (28.3 kg/kN/h)
Cruise: 0.545 lb/lbf/h (55.6 kg/kN/h)
Source: https://en.wikipedia.org/wiki/General_Electric_GE90#Specifications
I assume this is typical for a modern HBPR turbofan.
But why?
Of course, as altitude rises, the reduced air density means the fan has less airflow, causing less thrust to be created. But then again, the bypass ratio does not change, so the core airflow also decreases by the same amount as fan airflow. So fuel flow should be decreasing too! So why is the TSFC TWICE as bad for cruise as takeoff? Is it because of losses in the fan duct increasing rapidly with speed?
(By the way, I know the higher speed helps reduce the seat-mile consumption, but still.)
EDIT: Wait what the? RB211, another HBPR turbofan, has almost no difference between cruise and "sea level" https://en.wikipedia.org/wiki/Thrust-specific_fuel_consumption

So you're saying the percentage of annoying things like friction increase rapidly due to the lower cruise thrust?

Yes, that is typical for high bypass ratio (HBPR) turbofans.

But why?

Because the entry impulse goes up while the exit impulse stays roughly constant. Thrust is the difference between both, derived over time.
The moving engine needs to slow down the airflow for combustion to take place, and then needs to accelerate the air by more than it has been slowed down to have positive thrust. Hence, SFC goes up in parallel with speed.
While the internal process inside the engine in flight is very similar to that at rest (only the pressure levels are increased by the ram effect of the moving engine and density drops with increasing altitude), thrust is reduced due to the smaller impulse difference in flight. Since a HBPR engine accelerates most of the air volume streaming through it by only a little, the average exit speed is relatively low compared to a jet engine with no bypass flow. This is similar (but less severe) to a propeller where thrust drops with the inverse of speed.
SFC gets even worse in supersonic flight. For a meaningful SFC comparison speed needs to be the same.

Thrust-specific fuel consumption in g/kN·s of a turbofan engine with bypass ratio 3. Y axis shows altitude in Meters. Isolines wer made with R contour function and should be more rounded in reality.

But why does the exit velocity remain constant? it would seem that as speed increases, the energy being added to the air stays constant, so the exit velocity relative to the ground should be static exhaust velocity + airspeed.

If the fuel flow remains same, the energy released in the engine should be same, but as you say, the airflow is being accelerated less at cruise, therefore the energy consumed by that process would seem to be decreasing, giving a lot of engine power, which to me would seem to make the engine want to increase RPM

Or put another way, at a stanstill, the angle of attack to the fan blades is maximum. At cruise, it decreases at constant rpm.

And if you kept the RPM same and increased airspeed further, the fan becomes a turbine! And yet the engine continues to burn takeoff amounts of fuel?

@ABJX: The intake ensures that the angle of attack of the fan blades does not change much. Only the pressure level does.
The exit speed is limited by the maximum turbine temperature. Precompression heats the air entering the combustion chamber and raising the temperature to the maximum needs a bit less fuel but keeps the exit speed constant.

Oh yes I forgot. The fan has a ram intake.

But still, you're saying that the fuel flow decreases a "bit". But the velocity at the fan face remains constant, so the acceleration by the fan.........only decreases a bit. And this acceleration is added to the velocity the air gains from ram pressure release when it is expanded through the fan nozzle at cruise.

@ABJX: Flow speed is reduced to almost zero in the diffusor. The compressor does not accelerate the air, it only compresses it, hence the name. In order for combustion to take place, fuel and air need time to mix and a higher velocity would simply blow out the flame. Remember that typical flame front speeds for air-hydrocarbon mixtures is only a few meters per second.

I think you mistook the core compressor for the fan, which is what I was referring to. Though I must say that the core also seems to experience the same effect.

@ABJX: Ah, you mean the outer flow. Precompresison is drag – what is taken away is returned to the flow in the nozzle. Speed increase from the fan is roughly constant over flight speed.

Let us continue this discussion in chat.


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Are larger airplanes more likely to stall?
Stalls happen when the boundary layer on top of a wing slows so much that it essentially becomes part of the airfoil.

My question is, as the wings on larger aircraft present a longer flow path for the boundary layer, they slow the layer more. Right? So, would they not be more have a lower stall angle? Isn't this what is hinted at by the use of slots on airliner flaps?

I think this answser contains relevant elements (link between chord length, Reynolds number and stall angle of attack). Good answer should refer to these 3 concepts.

I think there are concepts to clarify. boundary layer does not move (it is a layer in which air molecules are affect by friction over the wing surface), and separation point is where flow is no longer laminar. For stall, several definitions exist depending if you speak of the critical AoA or the stalled regime.

@ManuH I think both the definitions you mentioned can be used to answer my question - a more stallable wing will get trailing-edge separation at lower AoA. But that's just what I'm guessing. I may be wrong.

@ManuH In the answer you linked, Peter said "generally a higher Reynolds number means your friction effects go down, translating into less friction drag and a higher stall angle of attack" which I don't get, because this implies that a slower,  boundary layer airflow is less likely to reverse (what?)

@Bianfable I've never once claimed to be a "knowledgeable" person at this site!! I've asked a ton of questions and, occasionally, I've managed to come up with a reasonable answer.

@ABJX I linked this answer in order to point out that some elements already exists on aviation.SE and answers to your question should bring new elements or bring them differently. If there is something you don't understand, don't hesitate to ask a new follow up questions about specific concepts you've just discover.

Even assuming the same airspeed, a larger chord would increase the free-stream Reynolds number. As a general trend, higher free-stream Reynolds number will delay the stall boundary due to earlier transition to a turbulent boundary layer, as can be seen below for a 2D section data from wind tunnel:

Image ref: NASA Contractor Report 4745
This answer has a different and equally good illustration of the effect in terms of lift curves.
Of course, things get more nuanced in 3D, whether the airfoil is optimized for natural laminar flow, and in transonic flow, where the maximum lift trend with Reynolds number can reverse at an intermediate Reynolds number.

So the transition to turbulent is the only reason why a higher chord/Reynolds gives a higher stall angle.

@ABJX For normal airfoil/wing, I believe so. But I can't be certain whether that's the only physical process. However, the correlation is observed time and time again in wind tunnel and in literature.

@ABJX: No, there is more. The boundary layer may be absolutely thicker than on a small wing, but is relatively thinner, so viscosity has less influence on the flow conditions. Also, larger aircraft tend to have higher wing loadings, so they fly faster which increases Reynolds number again and reduces the relative effect of gusts.

so there is flow separation at a lower AoA but it is less severe due to the relative thinness of the boundary layer?

@ABJX No, boundary layer separation will be delayed to a higher AOA. Furthermore, as Reynolds number increases, the laminar separation bubble will also get smaller for the same AOA.

wait then why use slotted flaps and slats? they cut up the wing into several smaller wings.

@ABJX If it's just slotted flap, the stall AOA actually decreases compared to plain airfoil, even though the CLmax has massively increased. As for slats, it definitely delays the stall AOA. My understanding is that a slat reduces the suction peak, thereby reducing the adverse gradient on recovery.

Any wing is 100% likely to stall when its critical angle of attack is exceeded, so no, larger wings are not more likely to stall than smaller wings. Both are certain to stall when the necessary condition is reached.
To address the edit in the question, are large aircraft more likely to stall... any aircraft will stall if it's flown to the point of exceeding the critical angle of attack.  Put it there, and just like a wing in isolation, the probability is 100%.  If the question is an empirical one, then all sorts of other factors come into play:
Small training aircraft are stalled routinely and intentionally as part of the training syllabus.  Large jets are almost never intentionally stalled (an FCF flight would be the rare exception, since training is done in the simulator), and there are multiple warning systems in them to help the pilots avoid such a condition.  Plus, with a crew of two pilots who are typically highly experienced, the likelihood of getting there inadvertently tends to go down considerably, when compared to a small aircraft that's often flown single-pilot by somebody who may not have extensive experience and may not fly as often as those who are paid to fly the larger jets.
To compare the AOA where the clean (no high-lift devices deployed) wing on a transport category aircraft stalls to the AOA where the wing on a light aircraft stalls is an academic exercise, but it doesn't relate to "is this aircraft more likely to stall than that aircraft."  Both will fly fine when operated in the normal envelope, and both will stall when flown to that point.

Excellent point.  Perhaps the OP should refine the question:  i.e.  "would the same airfoil shape, scaled up, stall at a lower critical AoA?"

I guess I should rephrase the question.

-1  This isn't an answer, it's a put-down.  Is the critical angle of attack more likely to be exceeded in a large aircraft?

@DaveGremlin  That's a whole different question.  I've exceeded the critical angle of attack dozens, probably hundreds of times in small aircraft, because it's common to practice stalls in training aircraft.  Never done it in thousands of hours in large aircraft, because that maneuver is reserved for the simulator in such aircraft.  Does that mean that one is "more likely to" stall the small aircraft?  Any aircraft can get there if you put it there, intentionally or not. Your version of the question has a behavioral aspect to it that the O.P.'s didn't.

@DaveGremlin  Also, the answer is NOT a put-down, but pointing out that the question as phrased doesn't really have the sort of answer that the OP may be searching for.  "Likely" is the wrong criterion.  Ask the question differently, and it gets a different answer.  But "same airfoil scaled up" or "big jet with its wing vs C-152 with its wing" or whatever other rephrasing of the question you have, is NOT what was asked.

@RalphJ  - Your original answer was: "Any wing is 100% likely to stall when its critical angle of attack is exceeded, so no, larger wings are not more likely to stall than smaller wings. Both are certain to stall when the necessary condition is reached."  This doesn't give the questioner anywhere to go and comes over as very dismissive.  People who ask questions on this site are not always experts in the subject, hence the question.  If there's a problem with the question it would be preferable to provide guidance  rather than two sentences that are desinged to illustrate the OPs ignorance

@DaveGremlin  I have no problem downvoting questions that I think are poor, but this question has zero DV. The problem with the "is likely" phrasing is that it's a behavioral, not an aerodynamic, issue. Aerodynamics is pretty deterministic. And, reference the OP's comment above, he understood the need to refine his question in order to get at what he was interested in.


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Would supersonic bi/tri/multiplane wings have less lift interference drag than a subsonic version?
Biplane wings interfere with each other, reducing the total lift often to about only 20% more than a mono wing of the same chord and span.
But at supersonic speeds - say, Mach 2, where Mach angle is 30 degrees, the wings (now provided with supersonic airfoils, before anyone asks) would have to be 0.5 chord-lengths apart for their shockwaves to interfere.
So, am I correct in saying that for a given chord-length separation, drag due to airfoil interference on vertically stacked multiplane wings interference decreases with Mach number?

Amazingly, at supersonic speeds it can work the other way. In the Busemann biplane the shock waves interfere in such a way as to actually reduce drag and also to reduce the sonic boom. Wide enough separation to avoid interference would actually increase supersonic drag.
See for example:

Wu, Jamieson and Wang
Kusunose, Matsushima and Maruyama

Yeah I knw about that one. I like it, though I heard it had shockwave-swallowing problems that needed variable geometry.

@Abdullah Not sure why you are asking if you already know the answer. perhaps you could clarify your question to be more precise in what you do want to know.

Well, I wasn't asking about a Busemann plane. More about 2 diamond airfoils stacked on top of one another.

Then I would suggest that yo edit your question accordingly.

ne wings have less lift interference drag than a subsonic versio

Busemann biplane - No shockwave, but no lift. Well, there's yer problem!

@FreeMan You are behind the times! Check out the references I provided in my answer.


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Do/why don't spaceplanes suffer from flutter?
Spaceplanes tend to have beefier, stiffer parts than normal aircraft, at least partly due to drag-reduction being less of a concern. But they also experience far higher airspeeds during reentry.
Yet I've never heard much, if anything, about flutter, or any other form of aeroelasticity being a problem on any of them. Maybe that's because I don't know much about existing or past spaceplane programs? But still, I'd be interested to know.
I can imagine some candidates for the reason why flutter might be less severe:

the frequencies are so high and modes so many and so uncoordinated that too much energy is dissipated as heat and sound, leading to simply vibration with no damage.
the sheer stiffness of the vehicle moves flutter frequency out of reach. (Which sounds rather far-fetched for me)

Hoping to find an answer, I Googled about aeroelasticity of the Shuttle, and got this document. It's rather long, so ATM I haven't drawn any conclusions from it.
It would be nice if someone has any background info on what the designers of the Shuttle (which I would guess is the best known spaceplane) faced during development in relation to flutter.

Hopefully someone who understands aerodynamics better will answer, but the premise seems wrong to me. The shuttle only experienced about 1.5g on reentry, which any airframe could handle. With stiff stubby delta wings, it seems like flutter would be unlikely to develop. I could be wrong though.

@BretCopeland 1.5G? I thought it was 3

3g during launch. There's certainly a lot of vibration during launch. All launch vehicles and payloads go through vibration analysis. I'm sure if there were any issues, they simply would have strengthened areas as necessary. My impression is that the opposite occurred - the Columbia/Challenger airframes were stronger than they needed to be and Discovery, Atlantis, and Endeavor benefited from weight reductions.

Do/why don't spaceplanes suffer from flutter? I think it should just be "Why don't spaceplanes suffer from flutter?". "Do don't spaceplanes suffer from flutter" doesn't sound quite right.

The old Orbiter Aerodynamic Design Data Book (https://archive.org/details/nasa_techdoc_19810067693) has lots of cool graphs showing aeroelastic effects on stability (Page A.1-7 fwd) but interpreting them is not something I can do. The whole of Appendix A is about aeroelasticity of the Orbiter.  I think it's fair to conclude that aeroelasticity was considered in the design.

The obvious answer would seem to be "because they're designed not to", per @Organic Marble's comment.  Just what goes into designing them (or indeed, any airplane) not to have flutter is material for an aerospace engineering degreee, not something that could really be answered here.

@BretCopeland fixed

Apologies for trying to offer an answer to a nearly three year old question, but I'm unable to resist this one.
The truth is that all spacecraft that operate (however briefly) in our atmosphere do need to be concerned with aeroelasticity. It is only through careful design, analysis, and testing that a vehicle will be free from catastrophic aeroelastic effects -- it certainly doesn't happen by accident.
As you noted in your link (NASA TM X-2570), there was an entire session devoted to aeroelastic effects on the space shuttle at a single joint NASA/AIAA conference in 1972. Slide 28 of the PDF is particularly interesting because it clearly shows that the biggest challenges for shuttle flutter analyses were in the transonic regime, not necessarily the hypersonic regime. This is largely true today and the explanation is probably telling. In exterior hypersonic flow, the forces are generally fairly steady and (on surfaces facing the freestream) typically very readily obtained (e.g., using Newtonian aerodynamics -- see NASA TM X-53391). Transonic aerodynamics, however, are highly nonlinear and required modest computing power to evaluate that didn't really become available until the late 1980s (e.g., Silva and Bennett, "Using Transonic Small Disturbance Theory for Predicting the Aeroelastic Stability of a Flexible Wind Tunnel Model", 1990).
If the question is really "why don't we see aeroelastic failures of space planes?", the answer comes back to all that careful analysis, design, and test. Space programs in particular are extremely thorough about working through every possible issue before flight. I've heard this derisively called "analysis-paralysis" and some of the criticism leveled at the time and budget required for space vehicles may indeed be fair. However, unlike their air-vehicle counterparts, which have the ability to slowly expand a flight envelope guided by analysis during flight testing, space vehicles are usually an all-or-nothing. Thus, there is a ton of time, man-hours, and money that go into ensuring that these vehicles are flutter-free.
Having said all that, it's not that space vehicles are actually so well understood that they never suffer from unexpected aeroelastic problems. Indeed, it's not uncommon that they do encounter these types of issues as they transition from static, to low-speed, compressible, transonic, supersonic, and finally hypersonic flight regimes (or vice-versa on entry). Thankfully though, the issues encountered have historically not been catastrophic (did I mention the enormous amount of effort that went into design, analysis, and test?). However, when these issues do crop up, they are assiduously examined post-flight and the vehicle typically will not fly again until the issue is expected to be resolved.

As simple as that, good answer

Not Shuttle specific, but there are generally two ways to prevent flutter of a control surface; for a manually operated surface, you balance it to put the surface's CG at or slightly forward of its hinge line, so that motions of the parent surface have a neutral (if CG on the hinge line) or counteracting (if CG is forward of the hinge line) influence on exciting sympathetic motion in the surface (that is, if the wing flexes up, with the CG aft the surface it "trails" the motion and displaces down relative to the wing attachment, putting it on the hinge line neutralizes this effect, and putting it forward of the hinge line has the opposite effect, creating an inertial moment in the same direction as the wing motion).
If the surface is hydraulically powered using irreversible hydraulic actuators, balancing of the surface generally isn't required and flutter protection is from the rigidity of the hydraulic connection (both in holding the surface from moving, and also acting as a passive damper when unpressurized). You can usually tell an airplane with irreversible hydraulics because there are no tabs or balance horns, or offset hinge lines, and the center of gravity of the surface will be some distance aft of the hinge axis.
So jets with irreversible hydraulic controls typically have unbalanced control surfaces that depend on the hydraulic actuators themselves for flutter prevention, and may also require additional passive damper units if the number of actuators used doesn't meet the minimum damping requirements following loss of an actuator. Service backlash limits for surface hinge attachments and actuator connections (resulting in surface free play) are largely a function of maintaining the rigidity of the system for flutter resistance.
The Shuttle having hydraulically driven control surfaces, you can be sure that flutter resistance is ultimately coming from the hydraulic actuators and the stiffness of the connections and structure (I don't know if the shuttle uses dedicated flutter dampers to back up the actuators).

Is this about the difference between free-floating vs non-free-floating surfaces?

I'm describing how you prevent flutter in a non-hydraulic surface, one operated by cables from a control column, or operated by a servo tab on the trailing edge of the surface and can be moved by air loads, vs one with hydraulically powered surfaces (irreversible hydraulics refers to the fact that airloads can't back drive the surface) where balancing is not required, and inferring that a craft like the Shuttle with hydraulically driven surfaces, will get its flutter resistance from the "rigid" hydraulic connection and not by mass balancing. That, and basic structural stiffness, is about it.

How does this prevent the wings from fluttering, though?

The control surface is not able to amplify motions in the main flying surface. Then you only have to make the main surface stiff enough to resist oscillating on its own.


text:
On large aircraft, why do flaperons not run the full length?

This is a 777 flaperon. It looks rather tiny, compared to the flap.
There have been air disasters where the airplane rolled uncontrollably for one reason or the other. It seems to me that a larger roll control authority could have helped.
Admittedly, flaps are slow, and speed-limited, so less suitable as control surfaces. But at least, could they not be used for trim?

Which disasters do you mean? I can't think of any where larger aileron authority would actually help.

This flaperon is actually the high speed aileron. Using the outer ailerons at high speed will twist the wing, which in turn reduces their effectiveness. For every aileron there exists a speed where it becomes ineffective and above that speed its function will even be reversed. See the flexible wing as a flap in its own and the aileron as its Flettner tab of sorts.
By using those high speed ailerons, Boeing can build the wing lighter with less torsional stiffness. In addition, it creates a gap between the Fowler flaps at the spanwise location of the engine exhaust, so the Fowler flaps will not reach into the very turbulent exhaust stream. The 737 uses an exhaust gate for the same purpose.
Using flaps for trim means you need to be able to finely adjust the position of each flap individually. This fine adjustment is better left to the smaller control surfaces or the even smaller trim tabs.

ah, aileron reversal. btw, surely we need more than a fine adjustment after losing part of our wing or something?

What he means is if the wing is torsionally flexible enough, the aileron in effect turns into a wing warping servo tab.  Imagine the Wright Flyer using a little servo tab surface at the wing trailing edge at the tip to warp the wing for roll control instead of cables connected to the pilot. Tab goes down, and instead of rolling airplane left, it warps wing LE down by lifting the TE, and the airplane rolls right.

@JohnK I knww. I mentioned it above

I've never seen it and I can't imagine the rolling moment you could generate would be worth the trouble.  You would have to have much larger surfaces in order to be able to combine pitch and roll inputs and still have the same overall pitch authority and the weight and complexity issues quickly kill any business case for trying it.

@Abdullah: Losing part of a wing is hardly a common occurence.  Why would one design for that, rather than designing wings not to fail?

@jamesqf design not to fail?

@Abdullah: Yes, you make the wing strong enough so that you don't have pieces falling off.  And if you did somehow lose part of the wing - say a midair collision, or someone shooting at the plane - having one control surface running the full length would probably mean it would either jam up or be torn off.  Whereas in the example photo, if you lose the part with the aileron, you still have a flaperon available.

@jamesqf well, we don't need it to be one monolithic surface.

@jamesqf the keyword here is authority. I want powerful backup control surfaces

@Abdullah: Then your best bet is to make a few billion dollars, hire some aircraft designers, and tell them that's the way you want your plane built :-)

Only simple flaps can be flaperons. Fowler and double-slotted flaps generally used on large aircraft cannot move upward, and are designed to extend slowly, which is more appropriate for their complexity and fairly large effect. But that's opposite of what you need in aileron, which must react quickly and using bigger deflection up than down is desirable to reduce the adverse yaw.
Normally in large aircraft roll control is augmented by using spoilers. These have the advantage that they create proverse rather than adverse yaw (the spoiler is raised on the inside wing and adds drag there) and don't suffer reversal at high speed as they don't torsion the wing. B777 is somewhat unusual in having a flaperon at all.

There have been air disasters where the airplane rolled uncontrollably for one reason or the other.

I can't remember any other reason than impeding stall, and in impeding stall the ailerons are reversed because deflecting the aileron pushes the wing deeper into stall and makes it go down rather than up. In impeding stall, the appropriate way to keep wings level is using rudder and yaw-roll coupling, which the pilots know.
Oh, and then I remember the Embraer E190 incident at Alverca (i.e. they managed to get it under control and land safely) where the control cables were connected crossed during maintenance, but in that case they were saved by the ailerons not having that much authority and roll control also employing spoilers which were still connected correctly and cancelled the ailerons at larger deflection.

The DC-7's left wing to the left side of the number one engine was mangled by the impact and was no longer capable of producing substantial lift. The engine had been severely damaged as well, and the combined loss of lift and propulsion left the crippled airliner in a rapidly descending left spiral from which recovery was impossible. et al

@Abdullah What flight is your quote above from?

@TomMcW this

@Abdullah, it's unlikely that larger aileron authority would help in any of such accidents. Loss of part of the wing is too much for an airliner, and such collision would also sever the hydraulic lines, so the remaining aileron would not be controllable anyway.


text:
Is co-flow jet wing technology workable?
Coflow Jet claims that their idea of sucking air from the trailing edge and blowing it out the leading edge improves efficiency.
But this looks like nothing but flow control done wrong to me. And by blowing at the leading edge, they give up on achieving laminar flow(!).
They also claim that it increases transonic efficiency.
Does this work?

Interesting concept. I doubt their assertion that pumping air from back to front uses little energy. Also questionable is the selection of references in their PDF: In all papers the author of the PDF is a coauthor. I guess it works for them if enough gullible investors chip in.

First red flag: a patent number in huge font on the first page, as if that proves anything (it doesn't).

Amazing! British pioneer JW Dunne proposed this as a propulsion system around 1901 but Sir Hiram Maxim told him it would not work. (ref. unpublished documents in the Science Museum Archive's Dunne collection). Nine years later Henri Coanda built a ducted "jet" plane on broadly similar principles, which failed miserably (The Wikipedia article is not very good, but I know of none better online). But they all failed to understand the opportunities for suction lift, instead trying to exploit it to entrain a greater volume of air and create thrust.
More modern blown-wing systems have tried to improve on suction lift using similar methods, but have always foundered on the power requirements for the blower and structural weight penalties of all that wing ducting.
Here is a PDF from their web site. It appears to achieve high lift/STOL performance through high AoA and the Coanda effect, basically turning itself into a giant blown flap. Alternatively it provides modest forward thrust by stimulating circulation and thus maintaining lift at a slight negative AoA.
Well, it probably does work - see wind tunnel images - but I would also want to see those wing weights, blower power figures and cost-benefit analyses before believing it is any more practicable than yesterday's approach.

I still can't get the impression of "flow control done wrong" off me

"But they failed to understand the opportunities for suction lift, instead of trying to exploit it to entrain a greater volume of air".  (Over a greater area of wing).  Slots and slotted flaps seem to do the same thing.  "Super suction" of migratory bird = lowering slats (lowering AOA too) on a fixed wing.  Very well informed, but nothing terribly new here, with benefits only needed at high AOA.  Very good + answer.


text:
Is night vision essential to pilots?
If someone with no night vision were in the cockpit of a nocturnal flight, that person's situation would resemble Instrument Flight Rules. Am I correct? So:

In theory, can someone lacking night vision operate an aircraft with no more danger than a pilot on IFR?

Can someone lacking night vision get a pilot's licence?

An answer that is valid for India, China, EU or the US is appreciated

You still have to see to take off, land, and do ground operations.

Are there people with “no night vision” as a permanent condition?  I’ve never heard of that, but if so, I’d expect it to be handled by a limitation on their medical certificate.

If your vision is so bad that you can not see at night, then it would also be compromised during the day, and you could not hold a medical certificate. This question should be closed.

@MikeSowsun How would it be compromised in the day?

“Night blindness” itself is a symptom of a number of common vision problems rather than a condition in and of itself. Some of those conditions are:
Cataracts,
Myopia,
Glaucoma,
Vitamin A deficiency,
Retinitis pigmentosa,
Usher syndrome.
https://www.vsp.com/eyewear-wellness/ask-eye-doctor/night-blindness

Adding to Mike S's comment and Ymb1's answer, "night vision" isn't a binary condition. (i.e. either you have it or you don't...)  Your ability to discern runway lights and other things is certainly important, but the degree to which you are able to meet applicable standards could vary depending on class of medical certificate, as well as country.  I think the question is overly broad...

@MichealHall the question says no night vision

@Abdullah, Yes, I see that.  So, if a pilot has no vision during the period of time defined as "night" per regulations, how would they even see the instruments?  I am being facetious to make a point here, that point being that you haven't defined "night vision" in any quantifiable terms.  The degree to which someone with degraded vision of any sort may or may not function safely, and may or may not qualify for a medical certificate can only be determined by a qualified doctor.  Again, there is a wide array of possible conditions and standards, so the question is overly broad and simplistic.

@MichealHall ouch. Night vision = the normal ability to adjust to low light conditions, such as when outside at might

Good grief, I understand what "night vision" is, but you do understand that what I mean by binary don't you?  A person's ability to see at night is likely to vary between individuals, and can even vary from night to night depending on many factors such as rest, nutrition, hydration, and oxygen levels.  O2 levels especially affect night vision, it is noticeable and immediate!!

@MichealHall I know what binary means. But I disagree with the Idea that this question is too broad

This answer is valid for EASA jurisdiction.

If someone with no night vision were in the cockpit of a nocturnal flight, that person's situation would resemble Instrument Flight Rules. Am I correct?

Only to some minor extent, because at night VMC, you still see a lot around you. Surface lights, quite often horizon line, sometimes backlit layer of clouds above you etc. You have some kind of perspective around you, limited, comparing to day VMC, but much less disorienting than IMC. This is why night rating requires 5 hours of practice and some theory (8 hrs, IIRC) whilst instrument rating (IR) minimum is 45 hours, plus about 4 months of theory (talking about PPL ratings now).
In EASA member states at least, night rating and instrument rating are not dependent on each other. You can acquire night rating only or IR only. Or both.

In theory, can someone lacking night vision operate an aircraft with no more danger than a pilot on IFR?

There is no «night vision» at least in EASA terminology. If you are referring to VCL mark (limitation to fly during daytime hours only + civil twilight) in your medical, then you are probably not color safe (like I am). Color blindness, mild or severe, as far as I know, is the most frequent reason for getting VCL mark.
EASA officials state that in dark environments, color vision is essential, especially recognition of pale colors. In US, for example, pilots that fail Ishihara, Anomaloscopy and CAD tests can still be granted a SODA (Statement of Demonstrated Ability) if they show their color recognition in practice. In Australia, color blind people can fly almost without any restrictions.
So the question is somewhat irrelevant, because a person with VCL mark («lacking night vision», as you say) can be IR rated and can fly IFR during daytime, officially and safely.

Can someone lacking night vision get a pilot's licence?

Absolutely, I am not color safe and have PPL pilot license, acquiring IR now to become fully instrumental.
However, with VCL mark, you will not be able to fly as a professional pilot (CPL or ATPL) at least in EASA jurisdiction, because VCL mark is only applicable to Class 2 medical. Class 1 medical has no tolerance towards conditions that result in your VCL limitation in Class 2 medical. Not being «color safe», you won't get Class 1 medical, which is required for CPL/ATPL.
And to complete the story, some countries require only regular driver's license medical certificate to issue ultralight class B (airplanes lighter that 472.5kg, like beautiful Evektor Eurostar, for example) license. But ultralights fly in Day VMC only, so the whole night vision discussion is irrelevant in context of ULAs.

US-based answer, and I'm not a doctor.
I don't know what "no night vision" actually means. But let's assume you mean someone has some kind of difficulty seeing clearly in low lighting conditions (again, whatever that means). You seem to be asking two questions here: is night vision important for pilots; and can someone with limited night vision get a medical?
At least theoretically, night vision only matters to pilots who fly at night. I know several pilots who have daytime-only restrictions on their medicals because of limited color vision. I assume (but don't know) that if someone had a hypothetical condition that meant they could see perfectly during the day but not at night, the medical examiner could issue the same limitation:

Not valid for night flying or color signal control.

You suggested that a pilot with poor night vision could just fly on instruments at night, but they have to get the aircraft to the runway first. And takeoff and landing both require visual cues, including seeing airport lighting at night. If the pilot really has difficulties seeing at night, it isn't obvious how they could safely operate an aircraft on the ground, never mind in the air.
As for a medical, I couldn't find anything in the FAA's medical standards about low light vision specifically, but there is some general wording that seems like it could apply:

Congenital or acquired conditions (whether acute or chronic), of
either eye or adnexa that may interfere with visual functions

I have no idea how a medical examiner would address any issues. They might have to defer the case to the FAA for review.


text:
Why use indicated airspeed instead of dynamic pressure

It uses the difference between total pressure and static pressure, provided by the system, to either mechanically or electronically measure dynamic pressure. The dynamic pressure includes terms for both density and airspeed. Since the airspeed indicator cannot know the density, it is by design calibrated to assume the sea level standard atmospheric density when calculating airspeed.
...
However, at typical civilian operating speeds, the aircraft's aerodynamic structure responds to dynamic pressure alone, and the aircraft will perform the same when at the same dynamic pressure. Since it is this same dynamic pressure that drives the airspeed indicator, an aircraft will always, for example, stall at the published indicated airspeed (for the current configuration) regardless of density, altitude or true airspeed.[4]

https://en.wikipedia.org/wiki/Indicated_airspeed
So we are using this "indicates airspeed" not to fund out how fast we are going, but to find out how much force the airplane is experiencing. That sounds like something for dynamic pressure, not some misindicated airspeed.

Is there a question here?

More accurately it sounds like square root of dynamic pressure.

You certainly could have an aircraft with a 'q' meter that reported dynamic pressure in psf, psi, bar, Pa, mmHg, or whatever your chosen units are.  The airplane cares about q, not about V. Pilots like to think in familiar speed units.  MPH or KTS are familiar.  Perhaps they would become familiar with q in psf, but perhaps not. The biggest issue is the 'squared' (or square root, depending on your perspective) relationship between q and V makes some things unfamiliar.    Of course regulations are written for IAS, there are many human reasons to not change.

Because when we are traveling somewhere we want to know when we will arrive.  We can easily calculate that if we know how many nautical miles we are traveling each hour.  A pressure based unit of measure would be relatively meaningless to a pilot.

You have to convert IAS to TAS anyway, so it wouldn't matter too much how you do it. But back when aircraft flew low it was different, so there is a historical aspect.

@JanHudec I believe historical aspect is precisely why we use IAS. In my opinion, early aviators would have figured out that instead of directly using dynamic pressure, if they used its square root, it would give a value linear with speed. And afterall, people are generally more familiar with "speed" rather than "dynamic pressure" and so IAS becomes a convention. Maybe you can write your own answer on that :-)

And how exactly does misIndicated Airspeed tell us when we will arrive?

@Abdullah, what unit of measure do you propose, and what useful information would it convey?

Pa? Psi? Unlike (KmIAS) Knots misIndicated Airspeed, it actually tells us why the aircraft is about stall or break up.

@Abdullah, and Vs or Vne is inadequate because why?  You state "not to find out how fast we are going, but to find out how much force the airplane is experiencing." but my point is that isn't true, we absolutely care how fast we are going.  I know that the faster I go the more drag force is in play, but I want an airspeed indicator to know what my airspeed is.  I don't really care about PSI, I can't convert that to a speed for time/distance calculations.  (FMI, does your use of "we" mean that you are a pilot also?)

P.S.  A dynamic pressure reading won't tell you anything about an impending stall, you need an AOA gauge for that.  And for predicting inflight breakup due to overstress, (other than exceeding Vne) a G meter.

@JanHudec, any aircraft simple enough to not display GS will probably be low enough that IAS is as good as TAS.  In any case, the point of my answer is that it is easier to get TAS from IAS than from PSI.

Airspeed is a calibrated value based on ram air pressure as collected by the pitot tube or other sensor minus static pressure, which is the ambient pressure of the air. If you don't subtract static pressure then your airspeed figures will be inaccurate. Think of it this way, an airplane parked in still air should have an airspeed of zero, if your airspeed indicator reads static pressure then the airspeed will have an indication despite there being no dynamic pressure. This inaccuracy will vary with changes in the static pressure, so you'll never really know what your airspeed is, which is critical for safety.

Yes, good point, thanks @Bianfable.

You seem to not comprehend the question. Dynamic pressure is total pressure minus static pressure.

I answered my interpretation of your question, I suggest you clarify the question so you get better answers. @Abdullah

It would be really interesting to see how a mechanical airspeed indicator makes the $\sqrt{P}$ adjustment.  Indicated is very close to true speed at sea level at standard conditions.


text:
Why is there conflicting nomenclature for the 1,000 ft runway markings?
The two thick solid lines on runways at 1,000 ft from the end of the threshold are called "fixed-distance markers" in some documentation, and in others, it is called the "touchdown zone marker".
Are they two different things, is one right and the other wrong, or are they truly interchangeable terms?
Airman Knowledge Testing Supplement for Instrument Rating, page 116

AIM 2-3-1 from FAA.gov

Both of these above examples are Precision Instrument Runways.

I understand the first down-vote because I submitted the question with an incomplete sentence, but why the second today?

First, let’s clear up some terminology. Touchdown zones are only on runways with precision approaches. Aiming Point Markings are at 1,000' in USA (ICAO standard is different and is runway length dependent) and are also on non-precsion and visual runways. If you fly a small GA airplane, you will often land in the first 1,000' using the Aiming Point as a guide. The markings are at a fixed distance (500') from each other so that’s probably why you have heard that term. I have most of the FAA publications and ACs on my computer and the only place that "fixed distance markers" is referenced is in the Chart Supplement for some runways that have non-standard distances for the markers, so I am curious as to where you have read that term. EDIT: Per the edited question, I changed my search to “fixed distance marker“ and found a half dozen references. All of them are in Knowledge Test questions and none in other FAA publications.
AIM 2−3−3. Runway Markings

e. Runway Touchdown Zone Markers. The touchdown zone markings identify
  the touchdown zone for landing operations and are coded to provide
  distance information in 500 feet (150m) increments. These markings
  consist of groups of one, two, and three rectangular bars
  symmetrically arranged in pairs about the runway centerline, as shown
  in FIG 2−3−1, Precision Instrument Runway Markings.

TOUCHDOWN ZONE− The first 3,000 feet of the runway beginning at the
  threshold. The area is used for determination of Touchdown Zone
  Elevation in the development of straight-in landing minimums for
  instrument approaches.

The place I have found it is in the FAA Instrument Testing Supplement. I will add it to the question.

@RyanMortensen I changed my search to "fixed distance marker", singular, and found it in several tests. (ATP, IRA, COM and CFI) but nowhere in the FAA publications. They are at a fixed distance (500') and I have referred to them as such, but apparently the FAA only uses that term on the tests.

The test answer for it is 1,000 ft from the beginning of the runway threshold, but 500 ft between it and each TDZ marker.

Aiming points are not required marks on non-precision runways below 4200ft with 3/4mile or greater visibility requirements, or any visual approach runways not serving CAT C&D aircraft and longer than 4200ft. FAA doc 5340-1m table 2-1